Stains on the pool surface will either be organic or inorganic in nature. Organic matter allowed to decompose will stain the surface.  A slight increase in the chlorine level will help bleach out this type of stain. Inorganic (metal) stains can vary in color depending on the particular metal. All water will have some metal content and oxidizers (chlorine) can cause these metals to stain our pool surfaces. Sometimes we inadvertently introduce metals into the pool water through metal heater parts, copper plumbing, metal based algaecides, fertilizer, etc. The best suggestion for stain control is the regular use of a metal sequestering agent. These chemicals help keep the metals in solution in the water so they don’t plate out on the pool surface causing a stain.